1. Field of the Invention
The present invention relates to a frame rate conversion apparatus and a frame rate conversion method for performing frame rate conversion of an input image.
2. Description of the Related Art
Display devices are roughly divided into two categories, impulse type and hold type, in terms of their display characteristics. Impulse-type display devices as referred to herein are display devices that have a short light emission time in a single frame, as illustrated in FIG. 1A. On the other hand, hold-type display devices are display devices such as liquid crystal panels that hold display in a single frame almost constant, as illustrated in FIG. 1B.
Examples of the impulse-type display devices include CRT (cathode ray tube) displays and field emission type displays. The impulse types have the characteristic of tending to cause flicker such as a screen that appears to be flashing because of repeated flashing of pixels. Since flicker is more easily detected with higher luminance or in a larger screen area, the problem of flicker with the impulse-type display devices further needs to be improved with the recent increase in display screen size.
One example of the hold-type display devices is an LCD (light crystal display). Because of a long pixel illumination time, the hold types have a problem in that a moving image causes motion blurring due to the characteristics of the hold types. In other words, the problem that needs to be improved is reduced visual resolution of moving images.
As a method for improving such motion blurring, a method has been suggested for adding an optical shutter mechanism to a liquid crystal panel so as to restrict a display light duration time and thereby bring display characteristics nearer to those of an impulse-type display device (see Japanese Patent Laid-open No. 9-325715, for example).
The above method (Japanese Patent Laid-open No. 9-325715), however, has a problem in that flicker occurs because display characteristics are closer to those of the impulse types where pixels repeat flashing.
One example of a method for reducing such flicker is a method for increasing the frame rate of display by dividing an input frame into multiple subframes by an arbitrary factor. For example, an input image illustrated in FIG. 2A is divided into two subframes as illustrated in FIG. 2B so as to double the frame rate of display, which increases the time frequency in display and, as a result, renders flicker less likely to be detected.
However, at the time of viewing such a display, a subframe that is behind in time deviates from the line-of-sight tracking during a single frame time as illustrated in FIG. 3, so pseudo contours that are dependent on visual characteristics arise.
In order to reduce such pseudo contours, a technique for increasing or decreasing spatial frequency components in a divided subframe at the time of frame rate conversion is known (see Japanese Patent Laid-Open No. 2002-351382, for example). With this technique, high frequency components in a subframe that is behind in time are attenuated and perceived pseudo contours are improved accordingly.
With the above-described technique (Japanese Patent Laid-Open No. 2002-351382), however, there is the problem of floating black levels whereby edge contrast in the vicinity of edges decreases, and the problem of the tailing of moving edges being noticeable. In the above-described technique (Japanese Patent Laid-Open No. 2002-351382), while an image is separated by the spatial frequency, the separation frequency is determined by the filter factor of a low-pass filter or a high-pass filter.
FIGS. 4A and 4B are diagrams illustrating the relationship between the filter factor of a low-pass filter and the output thereof. FIG. 4A represents spatial weights for the case where the filter factor of a low-pass filter is high, and FIG. 4B represents spatial weights for the case where the filter factor is low. It can be seen that pixel values that are farther away from the processed pixel position are weighed more in the case of a higher filter factor.
FIGS. 5A to 5C are diagrams illustrating outputs with respect to an input, for the case where the filter factor of a low-pass filter is high and the case where the filter factor is low. FIG. 5A represents the input. It can be seen that the high filter factor illustrated in FIG. 5B results in a lower spatial frequency than the low filter factor illustrated in FIG. 5C.
FIGS. 6A and 6B are diagrams illustrating the relationship between outlines of display outputs in the case of applying the above-described technique (Japanese Patent Laid-Open No. 2002-351382) and the appearance of the display outputs as captured visually. Referring to FIG. 6A, it is found that in a high edge-contrast region, the luminance in a black-filled region in the drawing increases and the region appears as floating black levels. A problem is that such a floating-black-level region, for example, an image-degraded region, increases in area as the filter factor increases.
Referring to FIG. 6B, on the other hand, it is found that edge breakups occur in edge portions of a motion region. Such breakups expand as the filter factor decreases.